1. Field of the Invention
The present invention relates to medical fabrics which are gamma radiation resistant, and to multiconstituent fibers for the preparation of such fabrics.
2. Description of Background and Other Information
An objective, in the nonwoven fabrics industry, is to produce such fabrics relatively inexpensively, while also satisfying one or more particular criteria. The nonwoven fabrics, such as those prepared by card and bond or spunbonding processes, in particular represent an economical class of fabrics, for the medical and related fields. Polypropylene fibers are conventionally used for preparing nonwoven fabrics, such as by the foregoing processes, due to the ability of polypropylene to thermally bond over a broad temperature range, and because polypropylene fiber can be carded into light webs at high speeds. However, exposure to gamma radiation causes considerable mechanical property deterioration to polypropylene; not only is such deterioration effected upon an exposure, but the deterioration from that exposure even continues, over the course of time. Gamma radiation treatment is a preferred method of sterilization in the medical and related fields, and is customarily used for all manner of medical fabrics and materials, including surgical and protective items. For this reason, polypropylene is disadvantageous for medical and related applications.
Like polypropylene, polyethylene is also a relatively inexpensive polyolefin. Polyethylenes have yet additional advantages, as set forth below.
For instance, in contrast to the polypropylenes, as discussed above, polyethylenes generally do not undergo extensive deterioration upon exposure to the dosages of gamma radiation which are employed for sterilizing medical items. Polyethylene fabrics have other favorable attributes, including soft hand, good drape, and heat sealability to polyethylene films; yet additionally, polyethylene is also widely recognized for its relative chemical inertness, especially its resistance to acidic or alkaline conditions, in comparison with polyester or nylon fibers.
However, melt spun polyethylene is rarely considered as a thermal bonding fiber, because it lacks the strong bonding property generally attainable with polypropylene fiber, and because of its lower fiber tensile strength. Polyethylene forms fibers which are slick, and of low modulus--generally, lower modulus than that of other types of staple fiber. Foremost among the difficulties normally encountered, in the production of thermally bonded polyethylene fabrics are the problems associated with carding the fibers--by virtue of their being slick and of low modulus, as indicated--and their lack of a broad thermal bonding window. Usually, polyethylene sticks to the calender roll before significant bonding can be achieved.
There is discussion, in the art, of 100% linear low density polyethylene fiber characterized by good bonding behavior. However, in such instances, the foregoing problems are avoided by handcarding the fibers, and bonding them at very slow rates.
Multiconstituent fibers having polyethylene as the continuous phase, with polypropylene dispersed therein, are known in the art. U.S. Pat. No. 4,634,739 (VASSILATOS '739,), and U.S. Pat. No. 4,632,861 (VASSILATOS '861, a division of VASSILATOS '739), disclose improvements to high pressure low density polyethylene (LDPE), obtained by the incorporation of polypropylene; however, the high pressure low density polyethylene, as disclosed in these two patents, is unsuitable for the preparation of a cardable, thermal bonding fiber.
U.S. Pat. No. 4,839,228 (JEZIC et al. '228), U.S. Pat. No. 5,133,917 (JEZIC et al. '917, a continuation of JEZIC et al. '228), disclose linear low density polyethylene (LLDPE) in combination blend with polypropylene, but with the blends obtained by use of a dynamic shear mixer, the use of which necessitates intimate dispersion, and, accordingly, domains of correspondingly small size. In this regard, these patents make particular reference to polyethylene fibrils dispersed in polypropylene fiber, with the diameter of fibrils near the fiber center indicated to be in the range of 350-500 angstroms (0.035-0.05 microns), and the diameter of the more populous fibrils, near the periphery of the fiber, being on the order of about 100-200 angstroms (0.01-0.02 microns).
Preparation of multiconstituent fibers, and of medical garments from such fibers, is likewise known in the art. U.S. Pat. No. 5,108,827 (GESSNER) discloses multiconstituent fibers, comprising a dominant continuous polymer phase and one or more discontinuous phases, with the former having a melting point substantially higher than that of the discontinuous phase polymer or polymers; GESSNER additionally teaches that fabrics prepared, from the multiconstituent fibers disclosed therein, are suitable for a variety of purposes, including use in medical garments.
However, GESSNER does not teach multiconstituent fibers with a polyethylene continuous phase. Further, GESSNER likewise teaches intensive mixing, and, therefore, the polymer domains which result must be correspondingly small, as is the case with the above-indicated JEZIC et al. patents.
It has been discovered that multiconstituent fibers which comprise a dominant continuous linear low density polyethylene phase and at least one discontinuous phase of poly(propylene-co-ethylene) copolymer and/or polypropylene--where the polymers are provided in the proper proportions, and where the one or more discontinuous phases are dispersed in domains of the requisite size--retain both the relatively strong bonding properties and cardability which characterize polypropylene, and also the indicated favorable attributes of polyethylene. Particularly, it has been discovered that fabrics prepared from such fibers have sufficient of the gamma radiation resistance and thermal bond strength which characterizes polyethylene, to render them suitable for medical and related applications.